A. Field of the Invention
The present invention relates to the field of X-ray radiography, in which ionizing radiation such as X-rays or gamma rays are used to form images of internal features of a human body, or of objects within luggage, shipping containers and the like. More particularly, the invention relates to a method and apparatus for reducing the intensity of X-rays back-scattered from backstops onto detectors used in X-ray radiography apparatus, thereby enhancing the signal-to-noise ratio and quality of radiographic images obtained by the apparatus.
B. Description of Background Art
Electromagnetic radiation of wavelengths substantially shorter than visible light, specifically X-rays and gamma rays having wavelengths less than about 0.1 nm, are routinely used to obtain visually discernible images of internal or sub-surface features of an object, by a method referred to as radiography. A widely used method of obtaining X-ray images, called transmission X-radiography, has been in use since shortly after the discovery of X-rays to obtain visual images of internal features of human bodies, such as bones and organs. Transmission X-ray radiographic images are created by exposing an X-ray sensitive device to X-ray radiation which has been transmitted through an object from a source of X-ray radiation. Historically, photographic film plates were among the first X-ray sensitive devices used for X-ray imaging, and are still widely used in the medical and dental fields. Transmission X-ray radiography systems using photographic film plates employ an X-ray source such as an X-ray tube which emits a beam of X-ray radiation. The X-ray beam, which typically has a conical or fan shape, irradiates an object field and a film plate holder located behind the object field. An object such as a human being or selected portion of the human's body is positioned in the object field between the X-ray source and the film plate holder, and upon being suitably positioned relative to the film plate holder and X-ray source, held stationary at that position. The X-ray source is then momentarily energized for a relative short, e.g., one-second time period which has been calculated to be just sufficiently long to form an adequate image in the emulsion of a photographic plate held in the plate holder. The exposure time is kept as short as possible because X-radiation has a cumulative destructive affect on biological cells. Therefore, the dosage energy, which is proportional to the product of X-ray intensity multiplied by exposure time, is desirably kept as small as possible.
Some X-ray radiation which irradiates an object such as a human body is transmitted with little attenuation, while rays which impinge on denser internal parts of the body, such as bone, are more heavily absorbed or scattered, thus forming in the emulsion of the photographic plate a shadow image of the denser object features. The film plate is developed and fixed by conventional film processing chemistry reactions and is kept as a permanent visual record for viewing and analysis by medical professionals. One variation of transmission X-radiography, called X-ray fluoroscopy, utilizes in place of a film holder, a screen which visibly fluoresces in response to X-radiation, enabling real-time dynamic viewing of internal object features.
Another variation of transmission X-radiography utilizes in place of a film plate or fluorescent screen a matrix array of photodetectors which are overlain by a fluorescent scintillator material that produces flashes of light or scintillations when irradiated by X-ray-radiation. Electrical signals output from the photodetectors are amplified and processed to form an electronic image of X-ray radiation incident upon the photodetectors. The electronic image can be converted to a visual image on a visual display device such as a cathode ray tube (CRT) or Liquid Crystal Display (LCD) display device of a television or computer monitor. The electronic image can also be input to a computer which uses display recognition software to automatically recognize contraband such as guns or explosive devices hidden in luggage examined by an X-ray radiography system.
To reduce the dosage of radiation on an object, some newer X-radiography systems uses a collimator made of an X-ray absorbing material such as steel, which has a slit-shaped aperture that deforms a conically-shaped X-ray beam into a relatively thin, vertically elongated, wedge-shaped fan beam of X-ray radiation. Such systems typically utilize a vertically disposed linear array consisting of one or more columns of scintillator-type X-ray detectors positioned at a detector plane located on the far side of an object field positioned between the collimated X-ray source and the target plane. A mechanism is used to cause the fan beam of X-ray radiation to horizontally scan the entire width or horizontal extent of an object to be imaged. One method for causing the fan beam of X-ray radiation to scan an object utilizes horizontal motion of the object, on a conveyor belt for example, to move the object relative to a fixed X-ray radiation source and detector array. Another scan method used in Computerized Axial Tomography (CAT) scanning utilizes rotation of the X-ray source and, synchronous orbital motion of the collimator and a flat or curved detector array, so that the beam remains at a fixed location on the detector array as the beam traverses the width of a stationary object located in the object field.
Whichever method is used to effect relative transverse motion between a fan beam of X-ray radiation on an object and a detector array, each instantaneous position of the beam on a particular column of detectors in the array produces detector output signals indicative of features in a single, narrow vertically disposed slice of the object. Thus, as the fan beam of X-ray radiation traverses the object and detector array, a sequence of electrical signals is output from the detectors. This sequence of output signals corresponding to a vertical stack of locations of an object, must be concatenated into a side-by-side arrangement of sequentially sampled vertical signal slices which represent a two-dimensional image of the object, or in the case of a CAT scan, a three-dimensional image of a human body.
Practical X-ray radiographic systems require a rather careful choice of X-ray energy levels used to image particular objects. Thus, the energy of X-ray radiation used to irradiate or illuminate an object to be imaged must be sufficiently large to penetrate denser features of an object. However, the X-ray energy levels should not be so large as to cause unneeded tissue damage to living subjects. Also, when high energy X-rays are required to penetrate and image dense objects such as cargo containers, vehicles, and the like, a problem can arise in using current X-ray radiographic systems for that purpose. That problem is scattered X-ray radiation which arises for reasons which will now be described.
When a beam of X-ray radiation impinges on matter, some of the X-ray photons are absorbed, and some are scattered, either in a forward direction, i.e., in the same direction as the incident beam, or backwards towards the source. Thus X-ray radiation having an energy of less than about 100 kV is typically absorbed by photo-electric absorption process. On the other hand, X-ray photons in the approximate energy range of about 100 KV to about MeV are elastically scattered by Thompson or Rayleigh scattering processes, or inelastically scattered by a Compton scattering process. For X-ray photon energies above about 1 MeV, the photons are absorbed by a pair-creation process in which electron-positron pairs are produced.
The absorption or scattering processes described above cause a beam of X-ray photons which impinges on a material object to be attenuated, thereby forming a shadow image of the object on a detector array positioned on an opposite side of the object from the X-ray beam source. The angles at which X-ray photons incident on atoms of the object are scattered, i.e., reflected or diffracted from an incident beam, depend on the material properties of the object and the angle of incidence. Moreover, X-ray photons which are inelastically scattered by the Compton scattering process undergo an energy loss. In a typical target object in which the arrangement of atoms is not uniform throughout the entire object, as they are in a single crystal, the scattered X-ray radiation often appears to be distributed in random directions. The effect can be visualized by shining a laser beam into a glass of water mixed with some milk, wherein light scattered in random directions from colloidal milk solids suspended in the solution cause the entire glass of solution to glow visibly when viewed from various directions.
Photons which are scattered in directions that have polar angles between plus and minus 90 degrees of the direction vector of an incident photo beam, i.e., those contained in a forward-facing hemisphere as viewed from a target and detector, are said to be forward scattered. Conversely, photons which have polar angles of 90 degrees to 180 degrees from the incident photon beam direction vector, i.e., those contained in a rearward-facing hemisphere, are said to be back-scattered. Both forward-scattered and back-scattered photons can degrade the quality of images obtained in X-ray radiographic systems, for reasons which will now be explained.
As described above, transmission X-ray radiographic systems utilize a source of X-ray radiation to irradiate or illuminate an object, and form an image of internal features of the object on a photographic film, fluoroscopic screen, or detector array. The image is formed as a result of more or less attenuation of incident X-ray radiation by more or less dense object features, thus causing fewer or more X-ray photons to impinge on a detector array plane and thereby resulting in an image having darker and lighter contrasting features, as manifested by fewer or more detected photons in the darker and lighter areas of the image. Therefore, it can be readily appreciated that X-ray photons which impinge on a detector from a location that is not on a common axis which joins a particular detector element to the source of X-ray radiation which illuminates an object feature, do not characterize the object featured, and therefore constitute noise, rather than a useful signal. Such noise signals will degrade the quality of an image formed using the signal detector output.
For example, since practical X-ray radiographic systems utilizing a conically-shaped flood beam or a wedge-shaped fan beam of X-ray radiation to illuminate an object to be imaged, there are in addition to a narrow bundle of rays which lie in a pencil beam along an optical axis between the X-ray radiation source and a particular detector, a significant number of off-axis rays which angle away from the optical axis and therefore irradiate object features other than the one aligned with the particular detector. Off-axis object features can forward-scatter off-axis X-ray radiation beams into a particular detector which defines an instant optical axis. The forward scattered rays which impinge upon that detector do not characterize an object feature on the instant optical axis defined by that detector, and therefore, produce image-degrading noise signals in that detector. The noise signals result in image degradation which can reduce discernability of dense image features. Signal-to-noise degradation caused by forward-scattered X-ray radiation is particularly acute when imaging dense objects. This is because the intensity of X-ray radiation transmitted through dense objects is inherently small, resulting in small detector signals, which can more readily be overwhelmed by noise signals produced by forward-scattered X-ray radiation.
To reduce the deleterious effects of forward-scattered X-ray radiation on detector signal-to-noise ratios and image quality, some high-end X-ray radiography systems such as computerized axial tomographic (CAT-Scan) systems employ an individual collimator positioned at the front of each X-ray radiation detector used in the imaging system. The collimators are typically constructed as tubes made of a dense, high atomic number (high-Z) material such as lead, positioned in front of and in axial alignment with individual detectors. Such collimators function as individual lens hoods or baffle tubes for each detector.
In certain X-ray radiography systems, the X-ray radiation source energy levels, detector thickness, and other parameters are chosen so that virtually all incident X-ray radiation is captured by the detectors. Therefore, in such systems, there is typically very little X-ray radiation which is able to bypass or leak through the scintillation detectors and reach the back wall of a detector box in which the detectors are located. Thus back-scattered X-ray radiation is usually not a problem in such systems. Back-scattered X-ray radiation is, however, problematic in high-energy X-ray radiography imaging systems, as will now be described.
In high-energy, high dosage X-ray radiography imaging systems, because of high material costs and other practical considerations, it is usually impractical to fabricate detectors with sufficient thickness to absorb all or most X-ray radiation incident upon the detectors. Therefore, detector arrays used in typical high-energy X-ray systems allow 30% to 40% of incident X-ray radiation to be transmitted through the detectors. Such systems are usually constructed so that the errant X-ray radiation which is transmitted through the detectors strikes a back-stop wall positioned behind the detector chamber, but some of that radiation is unavoidably scattered back towards the detectors.
In compact X-ray radiography systems, the back-stop wall is usually made of steel or lead which has a thickness sufficient to reduce the intensity of X-ray radiation that escapes from the detector chamber to a reasonably safe level. In large, fixed-site X-ray radiography systems, the back wall is typically made of concrete.
To reduce the effect of errant X-ray radiation which is back-scattered from a back wall towards the detectors, typical systems are designed with the back wall located many meters rearward of the detectors. There are, however, a variety of situations in which back-scattered X-ray radiation can impinge on X-ray detectors in a manner which causes significant degradation of the signal-to-noise ratio of detector output signals, resulting in significantly degraded images of object features obtained using the detectors, as will now be explained.
As described above, typical transmission radiography systems used to form images of internal features of an object utilize multiple detectors arranged in an array, which generally has a rectangular shape. The detector array is positioned some distance behind an object field containing an object, and the object field is positioned some distance behind a source of X-ray radiation, which typically illuminates the object field with a conically-shaped beam, or a wedge-shaped fan beam. Different parts of an object positioned in or passing through the object field may have very large variations in density, and therefore, in X-ray absorption or transmission characteristics. For example, in forming an X-ray radiography image of an automobile or other motor vehicle, X-ray radiation which passes through an engine block will be highly attenuated, while X-ray radiation which travels through open windows of the vehicle will be minimally attenuated.
Obviously, rays of X-ray radiation which have been transmitted through dense object features such as an engine block will retain a very small portion of incident intensity when impinging on a detector. Such rays will therefore result in very little energy being transmitted through the detector, and therefore will cause minimal back-scattered radiation from the back wall of the radiography site. However, incident X-ray radiation beams which are transmitted through low-density regions of the object will retain much of their energy when incident upon the detectors. Thus, even though some attenuation of these high energy rays occurs in the detector, a substantial portion of the energy is retained to impinge on the back wall. Back-scattered radiation produced in the back wall material from these relative high energy rays which have “leaked through” the detector can impinge on the rear surfaces of adjacent detectors, as well as the detector or detectors through which the radiation leaked. And, since the scintillator layer in each X-ray radiation detector produces a light scintillation regardless of the source direction of the X-ray photons on the scintillator layer, the photo-detector optically coupled to the scintillator layer of X-ray radiation detectors irradiated by scattered X-rays produces output signals which do not represent object information, and therefore constitute noise. That noise signal detracts from signals representing object information, therefore degrading the signal-to-noise ratio of signals output from detectors, for example, which are irradiated by back-scattered radiation emanating from a back-stop wall.
The present invention was conceived of to provide a method and apparatus for reducing the intensity of X-ray radiation back-scattered onto X-ray radiation detectors, to thereby reduce noise signals output from the detectors, improve signal-to-noise ratio, and enhance contrast ratio and detail resolution of images formed in X-ray radiation radiography systems.